The present invention relates to a conductive substrate for an electrophotographic photoconductor. More specifically, the present invention relates to a conductive substrate for an electrophotographic photoconductor having an aluminum oxide film on its surface. The present invention further relates to a manufacturing method of a conductive substrate for an electrophotographic photoconductor.
Electrophotography has developed in the field of the photostatic copiers. Recently, electrophotography has been applied to laser printing and the like. Since electrophotography is far superior than conventional impact printing in image quality, speed, and stillness, it has come to be employed widely in many devices. The conventional photoconductor installed in these devices is made of a conductive substrate having a photoconductive layer formed thereon.
The conventional conductive substrate consists of a conductive base having an undercoating layer formed thereon. Aluminum is widely used for the conductive base. Organic substances are widely used for the photoconductive layer.
The undercoating layer is formed from coating a layer of plastic, such as polyamide, onto the conductive substrate. In the alternative, the undercoating layer is formed by anodizing an oxide film onto the conductive substrate. The latter is widely used in photoconductors of high reliability, since oxide films are advantageous under environments of high temperature and high humidity. A conductive base soaked in an electrolytic solution is anodized. An oxide film is then formed on the conductive base.
Generally, the film thickness of the oxide film formed on the conductive base is ruled by the current density and the passing duration of the current, so long as the anodic current concentration is not exceeded.
Recent anodizing methods of aluminum include adjusting the configuration and spacing of an opposing electrode. Further methods devise a wave form of the current which makes the electrolytic solution foam, improving the circulation of the electrolytic solution. Such a method enables uniform distribution of the current over the entire surface of the aluminum base anode. The uniform distribution of current controls the thickness deviation of the oxide film within the range of .+-.1 .mu.m. This limit in the thickness deviation creates a photoconductor having excellent printing quality.
Referring to FIG. 2, a surface of a conductive aluminum base 2a of a conventional electrophotographic photoconductor is anodized to form an aluminum oxide film 3. Conductive substrate 1a for a photoconductor includes conductive aluminum base 2a and aluminum oxide film 3.
A charge generation layer 4a and a charge transport layer 4b are successively formed on a surface of conductive substrate 1a to give a photoconductive layer 4. Charge generation layer 4a absorbs light and generates free charges. Charge transport layer 4b receives and transports these free charges.
A semiconductor laser light having a wave length of 780 nm is widely used as a light source for a printer.
Referring to FIG. 3, the above-mentioned light, having a wavelength of 780 nm, is irradiated on a conventional electrophotographic photoconductor. Conductive substrate 1a has aluminum base 2a and aluminum oxide film 3. Photoconductive layer 4 has charge generation layer 4a and charge transport layer 4b on conductive substrate 1a.
A part of the semiconductor laser light, having a wave length of 780 nm, incident to a photoconductor indicated by an arrow L, reaches aluminum oxide film 3 without being absorbed by charge generation layer 4a. The light partially penetrates aluminum oxide film 3. The penetrated light is reflected at the boundary of aluminum base 2a and aluminum oxide film 3 (arrow A). A portion of the light does not penetrate aluminum oxide film 3. This portion is reflected at the boundary of charge generation layer 4a and aluminum oxide film 3 (arrow B).
Reflected lights A and B have the same single wavelength and are coherent. Light B interferes with light A in photoconductive layer 4, resulting in the generation of interference fringes due to thickness variations. These interference fringes cause irregular printing density.
Japanese Laid-open Patent Publication No.6-317921 and Japanese Laid-open Patent Publication No.7-301935 disclose means to prevent irregular printing density by controlling the generation of interference fringes. These reports propose to anodize aluminum using a current of changing wave form, allowing the light to scatter in the oxide film.
When various improved manufacturing methods, such as those mentioned in Japanese Laid-open Patent Publication No.6-317921 and Japanese Laid-open Patent Publication No.7-301935, are used to obtain an uniform current density, there is obtained an aluminum oxide film with small thickness deviations. However, when semiconductor laser light is irradiated onto the photoconductor, interference fringes are generated due to small thickness variations by an interference action as described in FIG. 3.
On the other hand, an aluminum oxide film exhibiting the above-mentioned effect of light scattering in the oxide film results in increased thickness deviation. This increase in thickness deviation leads to photoconductors having variations in their characteristics.